Frequency modulation



Aug. 10, 1943. e. USS'ELMAN FREQUENCY MODULATIQN Filed Aug. '30, 1941@SheetS-Sheet 1 INVENTOR ATTORNEY Aug. 10, 1943. e. L. USSELMAN 2 4FRE'QUENCY MODULATION 3 Filed Aug. 30, 1941 2 Sheets-Sheet 2 INVENTORATTORNEY Gear/gel. Q's 96nd]:

I Patented lit, l

. George Lindley Ussclman, Port Jeficrson, N. Y., assignor to RadioQorporation of America, a corporation Eelaware Appiicatlon August 3t,1941, Serial No. 408,982

( 8 Claims.

This application concerns a. new and improved means for generating waveenergy and modulating the frequency of the same in accordance withsignals such as, for example, telephony, teleg raphy, photo radio,television, etc.

Briefly, my system includes a pair of electron discharge devicesconnected in a push-pull oscillation generator circuit of relatively lowQ with phase displaced'feed back circuits and push-pull modulation ofthe tubes to thereby modulate the frequency of'the oscillationsgenerated. Pushpull modulators have various advantages over single tubecircuits because the modulation characteristic in a push-pu1l system islinear over a greater range than in a single tube system. Moreover,push-pull modulation with a balanced osccillator balances out amplitudemodulation leaving only the desired frequency modulation. Anotheradvantage is that fluctuations of power voltage which change electrondischarge tube electrode potentials compensate in the push-pullarrangement and do not cause undesired fre quency shifts or modulations,as would be the case in a single tube circuit. Other advantageswhichflow from the use of a simple balanced oscillator and modulator such asI have invented will appear in the following detailed descriptionthereof, and still others will be apparent to those skilled in the art.

In describing my invention in detail reference will be made to theattached drawings, wherein Figures 1 to 3 each show somewhatdiagrammatically the essential features of oscillation generators of thepush-pull type with push-pull modulation thereof to modulate thegenerated frequency. In Figure 1, the phase shiftingfeedback circuitsinclude respectively two inductive reactances and a capacity and aninductive reactance. In this modification a common bias resistance isused to supply the control grid bias. In Figure 2 the phase shiftingfeedback circuits include respectively a resistance and inductance and aresistance and capacity. In this modification the phase shiftingresistances also serve as the grid biasing resistances. In thearrangement of Fig- .ure 1 modulation is applied to the control grids,whereas in the arrangement of Figure 2 modulation is applied to thescreen grids. In Figure 3 the push-pull modulated oscillator issubstantially similar to the one shown in Figure 2, In Figure 3,however, I have shown in greater detail the modulation circuits whichinclude means for modulating by different types of signals.

Referring to Figure 1, tubes VI and V2 have their anodes 2 and 4.respectively, coupled in sistance.

push pull relation by a tank circuit comprising an inductance Li andcapacity C3, to which in= ductance Ll may be coupled an output circuitincluding an inductance L2. The screen grids of these tubes VI and V2are supplied with direct current potential as shown, and the anodes 2and c are supplied with positive potentials by way of the inductance Liand a lead 6. The control grid 3 of tube Vl'is coupled to a point on theinductance Li by way of coupling and blocking condenser C2 and phaseshifting inductance L and to the cathode by phase shifting cliche litincluding the tube Vl resistance between the arid and cathode inparallel to L3 and condenser Ct. A similar phase shifting feedbackcircuit 'comprising coupling condeuseuci, condenser C, and phaseshifting inductance L l including the tube resistance and condenser C5coupling a point. on inductance Li to the control grid it. The tocathode resistances of tubes Vi and V2 are in parallel with inductancesLi and Lt respec= tively. The control grid capacities of tubes Vi and V2respectively tune out the inductance of chokes L3 and L6 respectively.As a consequence the phase shifting means are actually 0 and tube V2grid resistance and and tube VI grid re- Blas for the control grids 8and it is supplied by source It and/or resistance R. Modulatingpotentials of the desired character are supplied from a source itthrough coupling means such as a transformer ld'to the control grids itand it in push-pull relation. v

Oscillations are generated in the tubes Vi and V2 and their circuits ina well-known manner, the oscillator circuits being, except fordiflerences pointed out hereinafter, conventional. The control grid 8 oftube VI is excited by feedback potentials from one side of the midpointof tank circuit Li, C3, while the control grid id of tube V2 is excitedby feedback from the other side of this tank circuit. Disregarding thefunction of the phase shifting reactances L and L3, and C and Lt, andthe tube resistances the circuit described provides a conventionalpush-pull oscillator. However, in my novel system the excitationvoltages fed backto grids 8 and it are shifted by phase shiftingcircuits L, etc. and (2, etc. The voltages fed to the grid 8 areretarded 78 and those fed to the grid ID are advanced As stated abovethe phase shifting means in the control grid circuit of tube VI areinductances L and L3 plus the tube resistance. The phaseshittlng meansin the control grid of tube V2 are condenser C and inductance 4 plus thetube resistance. Condensers Cl and C2 are large and have substantiallyno effect on the voltage passed thereby. Excitation feedback energyobtained through coupling condenser C2 from one end of the anode tankcircuit C3, LI is retarded in phase by phase shifting means L and L3with the tube resistance when it reaches the control grid 8 of tube VI.This energy is amplified and reversed in phase by tube VI and fed backto the opposite end of the tank circuit C3, LI. Excitation feedbackenergy obtained by way of coupling condenser CI from the opposite end ofthe anode tank circuit C3, LI is advanced by condenser C and inductanceL4 with the tube resistance when it reaches the control grid III of tubeV2. This latter energy is amplified and reversed in phase'by tube V2 andfed back to the lower or opposite end of the anode tank circuit C3, LI.

The control electrodes 8 and ID of tubes VI and V2 are modulated inpush-pull relation as stated above. When the tube VI has its controlgrid 8 modulated less negative to deliver more energy .to the anode tankcircuit LI, C3 and the grid I8 of tube V2 is modulated less positive ormore negative to deliver less energy to the anode tank circuit Ll, C3,then, since the phase of the energy delivered by tube VI is lagging andthat delivered by tube V2 is leading, the phase and frequency of theoscillations in tank circuit C3, LI will slow down to a lower frequency.Stating this another way, the tube VI is supplying the greater amount ofenergy to the tank circuit LI, C3 and this energy is lagging in phaseand frequency with respect to the smaller amount of energy supplied tothe tank circuit Ll, C3 by tube V2. As a consequence, the frequency ofthe energy in the tank circuit LI, C3 approaches the phase and frequencyof the energy supplied by tube VI. When during modulation tube V2 isbiased less negative and tube VI is biased more negative, tube V2 isdelivering more energy to the tank circuit and tube VI is deliveringless energy to the. tank circuit. As a consequence, since the energysuppliedby tube V2 is leading and that supplied by tube VI is lagging,the phase and frequency of the oscillations in tank circuit LI, C3 speedup to a higher frequency and more nearly approach the phase andfrequency of the energy supplied by the tube V2. The amount of thisfrequency deviation from the average carrier frequency is proportionalto the amplitude of the signal oscillations and the frequency ofdeviation or frequency swing is the same as the frequency of the signaloscillations. The output energy of the modulator is fed to othermodulators, stages not shown before it is delivered for use.

The arrangement of Figure 2 is similar to the arrangement of Figure 1except that the inductances L3 and L4 of Figure 1 are replaced'in Figure2 by phase shifting and biasing resistances RI and R2. In Figure 2, asource 20 supplies potential to the screen grid electrodes 22 and 24,and these electrodes are modulated in push-pull relation by potentialsfrom source I6 and coupling transformer I8. The operation of thearrangement of Figure 2, except for the differences noted above, issimilar to the operation of the arrangement of Figure 1.

In Figure 3, the oscillator comprising tubes VI and V2, the tank circuittherefor, and the phase shifting feedback circuits therefor are similarin many respects to the corresponding tubes and circuit arrangements ofFigures 1 and 2. In Figamplifiers, and/or frequency multiplier ure 3 thegrids 8 and I0 are tapped to points on the capacitive branch of the tankcircuit. To do this I provide a capacity potentiometer includingcondensers CI and C2 and condensers C6 and C! which have their adjacentterminals grounded. The control grid electrodes 8 and I0 may bemodulated by one type of signal, such as, for example, telephonysignals, while the screen grid electrodes may be modulated. Electrodes22 and 24 may be modulated'by another type of signals, such as, forexample, C. F. V. D., telegraphy signals, etc. v

For convenience in describing the system of Figure 3, I have designatedthe modulated oscillator as a unit A, the telephony modulating means asa unit E, and the other modulating means as a unit B. The unit Bcomprises a tripping circuit, as will be described more in detailhereinafter, which operates on the screen grids of the frequencymodulated oscillator A, and audio amplifier C operates on the controlgrids of the frequency modulated oscillator A. Ordinarily when eitherstage B or C is not in use, switches SI or S2 may be open, respectively,to prevent the operation of the stage not required. The opening ofswitch SI and S2 cuts off the cathode heater currents of the tubes. Ifthese means are not sufficient to prevent reaction by the stage not inuse, then the switches (X and Y) and (U and V) may one or the other bealso opened to cut out the stage not in use.

The tripping circuit or stage B is used to provide and insure squarewave balanced keying for the frequency modulator stage A. The incomingtelegraph signals may be peaky and not square wave form and the signalamplitude may not be constant. The tripping circuit B is used not onlyto square up the keying wave form but also to limit the signals toconstant amplitude. This provides a means for holding the transmittersignal frequency deviation to a certain adjustable limit.

The tripping circuit, which consists of tubes V3 and V4, and resistors30, 32, 34, 36, 38 and 40 is not new in the art since it was shown inFinch Patent 1,844,950. Since the manner in which a. tripping circuitfunctions has been described by others, I will describe only its use inconnection with this invention. However, some of the details in the waythat I use it here are through potentiometer 48. When sufficient nega- Itive mark potential is applied to 48 it overcomes the space bias effectand causes the circuit B to trip definitely to the mark condition. Whenmarking. potential is removed or is insufficient, the circuit B tripsdefinitely back to the spacing condition. In this way the space and markcharacters of the telegraph signals are made to operate the trippingcircuit. The square wave form, constant amplitude signals in thetripping circuit B are coupled to the frequency modulator stage Athrough potentiometers 52 and 54 by way of leads 56 and 58 to the screengrids of tubes VI and V2. The outer ends of resistances 52 and 54 areconnected to the anodesof tubes V3 and V4 in the tripping circuit. Theadjacent ends of resistances 52 and 5d are both grounded through acommon bypass condenser 55. The adjacent ends of resistances 52 and 54are also connected to potentiometer 5B for positive bias supplyfor thescreen grids of frequency modulator tubes VI and V2. It can be seen thatby moving the points connected to leads 56 and 58 in and out from theadjacent ends of potentiometers 52 and 54 various degrees of coujustingthe screen grid bias which would be somewhat dimcult to do otherwise.For instance, if variable signal coupling were to be obtained by tappingleads 55 and 58 on resistors 30 and 32, the positions of low couplingwould be the points of highest screen grid bias potentials for tubes VI3 former T2, by closing switch 80, affected the bias of stage'A a smallamount. No detrimental effects-were observed whether the switch 80 wasopen or closed. It should be noted that it is desirable to keep thevalue of resistors R3 and R5 low in comparison to that of RI and R2 andto have the secondary impedance of transformer T2 match the resistanceof R3 and R4.

potentiometer 60 fixes a lower limit on the positive bias potential forthe screen grids of tubes VI and V2 as the coupling is lowered. It maybe stated here that adjusting the coupling between the tripping circuitB and the frequency modulator A results in adjusting the amount oftransmitter signal frequency deviation.

The condenser 6 3 ii added to the circuit B has the efi'ect of slightlyrounding off the corner on one end of each square wave signal character.A more satisfactory way of rounding off the corners of the square wavesignal characters would be to connect a low pass filter in series witheach of the leads 56 and 58. The reason for rounding off the comers ofthe signal characters is to limit or reduce the frequency band widthcaused by harmonics.

The audio frequency or telephone amplifier E is more or lessconventional. However, the circuit here is designed to fit therequirements of the frequency modulator stage A. The resistors 10 and 72are connected across the secondary windings of transformer Ti to matchthe transformer output impedance to the tube input impedance and toprevent parasitic oscillations. Resistor M supplie 'grid-to-cathode biasby the cathode-to-ground resistor method. It may be noted that T2 is astep-down transformer. In the system operated a plate to linetransformer was used and the resistors R3 and R4 in stage A were each250 ohms, RI and R2 being 2500 ohms. This arrangement was necessarysince these resistors are a part of the phase shifting circuits of thfrequency modulator and because of the low operating frequency of themodulator which was about 100,000 cycles. At higher operatingfrequencies of the modulator stage A, such as 2,000,000 cycles persecond, condensers could be used in place of R3 and RA withoutappreciable audio signal distortion. It was noticed that shortcircuiting bypass condenser 18 in the secondary center tap connection oftrans- Although I have shown the feed back voltages "fed to the grids 8and It in Figures 1 and 2 as being taken from points on the inductivebranch of the tank circuit and in Figure 3 as being taken from thecapacitivebran'ch of the tank circuit it will be understood that in eachcase it may be taken from either the capacitive or inductive branch.

1. In a wave length modulation system, an oscillation generatorincluding a pairof electron discharge tubes each having an anode, acathode and a control grid, a single tank circuit havinginductance,couplings between spaced points on the inductance of said tank circuitand the 'anodes of the respective tubes, 9. coupling between a point onsaid tank circuit and the cathodes of the tubes, a phase retardinginductive reactance coupling the control grid of one of said tubes to apoint on the said tank circuit, a phase advancing capacitive reactancecoupling the control grid of the other of said tubes to a point on saidinductance of said tank circuit, said couplings being such thatoscillations are generated in said tubes and tank circuit, and means forcontrolling the gain of the tubes in push-pull relation in accordancewithmodulating potentials to thereby modulate the wave length of theoscillations generated. v

2. In a Wave length modulation system,. an oscillation generatorincluding a pair of electron discharge tubes each having an anode, acathode and two grids, a single tank circuit, couplings between thespaced points on said tank circuit and the anodes of the respectivetubes, a coupling between a point on said tank circuit and the cathodesof the tubes, phase shifting reactances cross coupling correspondinggrids of said tubes to points on said tank circuit, said couplingsproviding regeneration in said tubes to generate oscillations whichappear in said tank circuit two sources of modulating potentials, meansfor couto corresponding grids of said tubes, and separate means forcoupling the other of said source of modulating potentials to the othercorresponding grids of said tubes.

3. In a signalling system, an oscillation generator including a pair ofelectron discharge devices of the pentode type, a single tank circuitcoupling the anodes of said devices in push-pull relation, phaseshifting reactances cross coupling the control grids of said devicesto-said tank cirtrol grid, a single tank circuit coupling the an-- odesof said devices in push-pull relation, phase shifting reactances crosscoupling the control grids of said devices to said tank circuit inpushpull relation, phase shifting impedances coupling L the controlgrids of said devices to the cathodes of said devices wherebyoscillations are generated by said devices and tank circuit, and meansfor modulating the potential on the control grid electrodes of saiddevices in push-pull relation to thereby modulate "the wave length ofthe oscillations generated.

5. In a signalling system, an oscillation generator including a pair ofelectron discharge devices each having an anode, a cathode, and acontrol grid, a single tank circuit connecting the anodes of saiddevices in push-pull relation, phase shifting reactances crossconnecting the control grids of said devices to said tank circuit, andmeans for modulating the potentials on corresponding electrodes of saiddevices to thereby modulate the frequency of the oscillations generatedincluding a pair of tubes having input and output electrodes, impedancescoupling the output electrodes of said tubes to corresponding electrodesof said devices, a source of potential connected through keying means tothe input electrodes of one of said tubes, a source of signals coupledto the input electrodes of the other of said tubes and impedances crossconnecting the input and output electrodes of said pair of tubes.

6. In a system of the nature described, a source of varying potential, apair of electron discharge systems including at least two electronreceiving electrodes and two electron flow control electrodes, animpedance connected between the electron receiving electrodes,impedances cross connecting the electron flow control and electronreceiving el ectrodes, connections for applying a steady bias to one ofsaid electron flow control electrodes,. other connections for applyingsaid varying potential to said other electron flow control electrode,and means for deriving potentials produced in said first namedimpedance.

7. In a wave length modulator circuit, a pushpull tube generator havinga single tank circuit with the tube grids and anodes coupled in pushpullrelation thereby, a phase advancing reactance in the coupling betweensaid tankcircuit and an electrode of one of said tubes, 9. phaseretarding reactance in thecoupling between said tank circuit and thecorresponding electrode in the other of said tubes, and means formodulating the impedances of the tubes in push-pull relation inaccordance with signals to thereby modulate the length of theoscillations generated while maintaining substantially constant theamplitude thereof.

8. In a wave length modulation system, an oscillation generatorincluding a pair of electron discharge devices each having an anode, acathode, and control grid, a single tank circuit coupling the anodes ofsaid devices in push-pull relation, connections cross coupling thecontrol grids of said devices to said circuit in push-pull relation, aphase advancing reactance in the coupling between an electrode in onedevice -and said tank circuit, a phase retarding reactance in thecoupling between the corresponding electrode in the other device andsaid tank circuit, said couplings being such that oscillations aregenerated in said devices and tank circuit, and means for modulating thepotential on corresponding electrodes of said devices in push-pullrelation to thereby modulate the frequency of the oscillationsgenerated.

GEORGE LJNDLEY USSEIMAN.

